Brazos River bar; a study in the significance of grain size parameters

A bar on the Brazos River near Calvert, Texas, has been analyzed in order to determine the geologic meaning of certain grain size parameters and to study the behavior of the size fractions with transport. The bar consists of a strongly bimodal mixture of pebble gravel and medium to fine sand; there is a lack of material in the range of 0.5 to 2 mm, because the source does not supply particles of this size. The size distributions of the two modes, which were established in the parent deposits, are nearly invariant over the bar because the present environment of deposition only affects the relative proportions of the two modes, not the grain size properties of the modes themselves. Two proportions are most common; the sediment either contains no gravel or else contains about 60% gravel. Three sediment types with characteristic bedding features occur on the bar in constant stratigraphic order, with the coarsest at the base. Statistical analysis of the data is based on a series of grain size parameters modified from those of Inman (1952) to provide a more detailed coverage of non-normal size curves. Unimodal sediments have nearly normal curves as defined by their skewness and kurtosis. Non-normal kurtosis and skewness values are held to be the identifying characteristics of bimodal sediments even where such modes are not evident in frequency curves. The relative proportions of each mode define a systematic series of changes in numerical properties; mean size, standard deviation and skewness are shown to be linked in a helical trend, which is believed to be applicable to many other sedimentary suites. The equations of the helix may be characteristic of certain environments. Kurtosis values show rhythmic pulsations along the helix and are diagnostic of two-generation sediments.

Changes in nutrient structure of river-dominated coastal waters: stoichiometric nutrient balance and its consequences

We present an analysis of extensive nutrient data sets from two river-dominated coastal ecosystems, the northern Adriatic Sea and the northern Gulf of Mexico, demonstrating significant changes in surface nutrient ratios over a period of 30 years. The silicon:nitrogen ratios have decreased, indicating increased potential for silicon limitation. The nitrogen:phosphorus and the silicon:phosphorus ratios have also changed substantially, and the coastal nutrient structures have become more balanced and potentially less limiting for phytoplankton growth. It is likely that net phytoplankton productivity increased under these conditions and was accompanied by increasing bottom water hypoxia and major changes in community species composition. These findings support the hypothesis that increasing coastal eutrophication to date may be associated with stoichiometric nutrient balance, due to increasing potential for silicon limitation and decreasing potential for nitrogen and phosphorus limitation. On a worldwide basis, coastal ecosystems adjacent to rivers influenced by anthropogenic nutrient loads may experience similar alterations.

水蚀风蚀交错带小流域生态环境综合治理模式研究

:水蚀风蚀交错带是黄土高原侵蚀最严重的地区和黄河下游河床粗泥沙的主要来源区 ,其生态环境脆弱、治理难度大 ,加之晋陕蒙能源基地的大规模开发 ,也带来了新的环境问题。鉴于治黄和煤田开发及生态环境整治的紧迫需要 ,以水蚀风蚀交错带六道沟小流域为试区 ,分析研究了该区水蚀风蚀时空分布规律及脆弱生态环境特征 ,提出以防治水蚀和风蚀为中心 ,以提高生态经济效益和持续发展为目标 ,以基本农田优化结构和高效利用及植被建设为重点 ,建立具有防蚀固沙兼高效生态经济功能的大农业复合生态系统综合治理模式 ,取得了良好的社会与经济效益

大尺度流域不同地貌类型区水土保持减水效益分析

趋势分析结果显示 ,河源涧区、风沙区、丘陵区各代表流域年径流量均在 1970年前后发生一致性改变 ,即从 1970年以来开始有明显的减少趋势 ,平均减流幅度 (与 195 9～ 196 9年径流相比 )以河源涧区 (大理河 )和丘陵区 (小理河 )最大 ,分别为 36 .33%和 36 .2 1% ,风沙区 (海流兔河 )最小 (2 0 .6 1% )。减流幅度的大小是各类型区下垫面状况、水土保持措施和治理程度、降雨量变化等多种因素综合作用的结果。借助适合于黄土高原降雨 -产流特性的月水量平衡改进模型 ,计算天然状态下降雨应有的产流量与同期实测径流量求得减水效果。结果表明 ,3个地貌类型区 70年代的减水效益没有明显差异 ,而在 80年代减水效果差异显著 ,丘陵区 (小理河 ,2 4.99% ) >风沙区 (海流兔河 ,17.2 8% ) >河源涧区 (大理河 ,13.13% )。上述结论为定量评价黄土高原生态环境建设对黄河水资源及水环境演变的影响提供了数据基础。